The present invention relates to a method and apparatus for producing oxygen in which the production is carried out in accordance with a Claude cycle. More particularly the present invention relates to such a method and apparatus in which a gaseous oxygen product is produced at pressure through utilization of a mixing column. Even more particularly, the present invention relates to such a method and apparatus in which part of the refrigeration is supplied from the mixing column to effect a reduction in the required amount of compression.
Air is separated by cooling a filtered, compressed and purified air stream to a temperature suitable for its rectification. The air stream is introduced into a double column air separation unit employing higher and lower pressure columns. The air is rectified in the higher pressure column to produce an oxygen-enriched column bottom and a nitrogen-rich tower overhead. The oxygen-enriched column bottom is further refined in the lower pressure column to produce liquid oxygen as a column bottom and a nitrogen vapor tower overhead. In a Claude cycle, the incoming air stream is compressed to a pressure well above the pressure of the higher pressure column and is turboexpanded prior to its introduction into the higher pressure column. The turboexpansion of the air adds refrigeration to the process in order to compensate for thermodynamic irreversibility of the process, for instance, cold box warm end heat leakages. Moreover, in a Claude cycle excess refrigeration can be supplied to in turn increase liquid production.
If a gaseous oxygen product is to be produced, a stream of the liquid oxygen can be pumped to the delivery pressure. The thus pressurized liquid oxygen stream can be vaporized within the main heat exchanger against the cooling of a portion of the incoming air stream that has been boosted in pressure. Alternatively, an oxygen compressor (at added expense and risk) can be used to compress a product stream at the warm end of the main heat exchanger.
The advantage of the Claude cycle is that a large proportion of the effort involved can be dedicated to the production of liquid oxygen. The disadvantage is that the oxygen production is at the expenditure of the energy required to compress the incoming air stream above the higher pressure column pressure. This problem is exacerbated when a booster compressor is used in connection with vaporizing a liquid oxygen product within the main heat exchanger. As will be discussed, the present invention provides a modification in the Claude process so that a gaseous oxygen product can be produced at pressure with a lower expenditure of energy over a prior art Claude process.